In my opinion it is of the utmost importance that you have your unvented cylinder serviced every year, the cylinder has a number of safety devices that require to be checked for functionality and safety, you need to insure the inline strain filter is clean and clear, as this can cause a restriction in flow/performance and also cause unwanted noise from the combination valve, it is important to check the correct operation of the thermal cut off device and that it has been wired correctly and that the domestic hot water zone valve actually physically closes when the thermal cut off is activated, also the temperature and pressure relief valves need to be manually operated to check for correct operation and the discharge tundish and termination also need to be checked for correct operation, the domestic expansion vessel needs to checked for the correct pre charged pressure or to re establish the bubble on some types like the megaflo, also on some cylinders there may be a sacrificial anode which might need replacing, but on most modern cylinders these are no longer required, i suggest the other components also get checked at the same time, eg heating circulating pump, heating zone valves and automatic bypass, you may also have a system filling loop and some automatic air vents, it would be wise to have drain cocks and stop cocks checked for water tightness and also the operation of the immersion heater if you have one.
essentially you have a device in your house which under normal circumstances is totally safe and a really good bit of kit, but if the unthinkable should happen and a couple of the safety devices fail, then it would be a very dangerous situation, eg if the thermal cut off did not physically shut the energy source off that heats it up before it got to boiling point and turned the water to steam, and if the tprv which is the temperature and pressure relief valve was not working, then you would have a big pressure build up within the cylinder and it could possibly end in a rupture or worse.
I will leave a link to a video on youtube of a cylinder explosion,
if you have any concerns or would like me to come and service your cylinder please get in contact.
£60 for stand alone service of unvented.
£45 when linked with a boiler service. (boiler service is £45, special offer until 1st August 2017)
Andrew Tyler 07958248040